The present invention relates to a gas sensor having a gas sensing film formed of an oxide semiconductor material and covered by a gas-permeable protection layer.
Various types of gas sensors have been proposed, one of which is a so-called oxide semiconductor gas sensor that has a gas sensing film formed of an oxide semiconductor material such as tin oxide so as to change its electrical properties in response to concentration variations in measurement gas and thereby detect the presence (or absence) or concentration of the measurement gas. The oxide semiconductor gas sensor deteriorates in performance over time and lack performance stability when the gas sensing film is poisoned by silicon etc.
There are some conceivable ways to prevent the oxide semiconductor gas sensor from performance deterioration due to poisoning of the gas sensing film. One way is to pretreat the gas sensing film with some poisoning material such as silicon and change the sensor performance to some extent in advance of actual sensor use. It is however difficult to regulate the degree of such sensor prepoisoning treatment in a variety of sensor use environments and, even by sensor prepoisoning treatment, merely possible to prevent a large change in the sensor performance at the early stage of poisoning during actual sensor use. Another way is to apply a gas-permeable protection layer onto the gas sensing film and protect the gas sensing film from any poisoning material. Japanese Laid-Open Patent Publication No. 9-166567 (abbreviated as “JP9-166567A”) proposes one such type of gas sensor having a gas sensing film formed of tin oxide and a filter layer (protection layer) formed of a mixture of tin oxide and amorphous alumina so as to obtain improvements in poisoning resistance and secular performance stability and reduction in initial stabilization time. Japanese Patent No. 3171745 (abbreviated as “JP3171745B”) proposes another gas sensor having a gas sensing film formed of tin oxide and a coating layer (protection layer) formed of tungstic oxide, titanium oxide or zinc oxide etc. so as to show high sensitivity to odorous gas and low sensitivity to non-odorous gas and obtain S/N ratio improvement for accurate odor detection in a lower concentration range.